System information is divided into a Master Information Block (MIB), which conveys a limited number of essential and most frequently transmitted parameters that are needed to acquire other information from a cell, and a number of System Information Blocks (SIBs), which convey dynamic system information. The MIB defines the most essential physical layer information of a cell required to receive further system information. SystemInformationBlockType1 (SIB1) contains information relevant when evaluating if a UE is allowed to access a cell and defines scheduling of other SIBs. Other SIBs are sets of related system information.
Change of system information only occurs at specific radio frames that are, for example, in a modification period. System information may be transmitted a number of times with the same content within a modification period, as defined by its scheduling. The modification period boundaries may be defined by System frame Number (SFN) values for which SFN mod m=0, where m is the number of radio frames comprising the modification period. The modification period may be configured by system information.